WO2001073383A1 - Method and device for detecting and/or monitoring the level of a medium in a container - Google Patents

Abstract

The invention relates to a method and a device for detecting and/or monitoring the level of a medium in a container or for detecting the density of a medium in a container. The aim of the invention is to provide a method and a device which enable the level or the density of a medium to be reliably detected and/or monitored. According to the inventive device, at least one first mode and one second mode of the oscillations of the unit (2) that can oscillate are evaluated and a change in the mass is detected on the unit (2) according to the evaluated modes, whereby said unit can oscillate.

Description

 Method and device for determining and / or monitoring the fill level of a medium in a container

10. The invention relates to a method and a device for determining and / or monitoring the fill level of a medium in a container or for determining the density of a medium in a container according to the preamble of claims 1 and 10 respectively.

Devices with at least one oscillating element, so-called vibration detectors, for detecting or monitoring the fill level of a medium in a container have already become known. The vibrating element is usually at least one vibrating rod which is attached to a membrane. The membrane is an electro-mechanical converter, for. B. a piezoelectric element excited to vibrate. Due to the vibrations of the membrane, the vibrating element attached to the membrane also carries out vibrations.

Vibration detectors designed as level measuring devices take advantage of the effect that the oscillation frequency and the oscillation amplitude are dependent on the respective degree of coverage of the oscillating element: while the oscillating element can carry out its oscillations freely and undamped in air, it experiences a change in frequency and amplitude as soon as it is partially or completely immersed in the medium. On the basis of a predetermined change in frequency (usually the frequency is measured), a clear conclusion can be drawn that the predetermined fill level of the medium in the container has been reached. Level gauges are used primarily as overfill protection or for the purpose of pump idle protection.

In addition, the damping of the vibration of the vibrating element is also influenced by the respective density of the medium. Therefore, with a constant degree of coverage, there is a functional relationship to the density of the medium, so that vibration detectors are ideally suited for both level and density determination. In practice, the purpose is Monitoring and detection of the fill level or the density of the medium in the container, the vibrations of the membrane are recorded and converted into electrical reception signals by means of at least one piezo element.

The electrical received signals are then evaluated by evaluation electronics. In the case of level determination, the evaluation electronics monitor the oscillation frequency and / or the oscillation amplitude of the oscillation element and signal the status 'sensor covered' or 'sensor uncovered' as soon as the measured values fall below or exceed a predetermined reference value. A corresponding message to the operating personnel can be made optically and / or acoustically. Alternatively or additionally, a switching process is triggered; for example, an inlet or outlet valve on the container is opened or closed.

The aforementioned devices for measuring the level or density are used in a variety of industries, for example in chemistry, in the food industry or in water treatment. The range of monitored filling goods ranges from water to yoghurt, paints and varnishes to highly viscous filling goods such as honey or to highly foaming filling goods such as beer.

Problems in level or density measurement using vibration detectors are caused by the fact that there is no clear connection between a frequency change occurring and the degree of coverage or density of the medium. An essential disturbance variable, which, like the coupling to the mass of the medium, is noticeable in a shift in the resonance frequency, is the change in mass at the vibratable unit Unity, as well as caused by the corrosion of the vibrating rods. Depending on the type and degree of the change in mass, the most undesirable case can arise here that the sensor permanently reports 'covered' or 'uncovered' and thus reports that the predetermined fill level has been reached, although the limit fill level has not yet been reached. The same applies to density measurement: the wrong density of the medium is measured and displayed. The invention is based on the object of proposing a method and a device which permit reliable determination and / or monitoring of the fill level or the density of a medium.

With regard to the method, the object is achieved in that at least a first mode and a second mode of the oscillations of the oscillatable unit are evaluated and that a change in mass of the oscillatable unit is recognized on the basis of the evaluated modes. Although in the following, reference is often made only to a mass increase that occurs when the oscillating unit is formed, of course, comparable considerations apply to the mass loss, which among other things. can be a result of the corrosion of the vibrating rods.

The invention is based on the physical effect that different vibration modes form when the oscillatable unit is excited. The following section explains in more detail which different vibration modes occur in a vibration detector with, for example, paddle-shaped vibrating bars.

According to an advantageous development of the method according to the invention it is provided that modes are evaluated as the first and second modes, the vibrations of which are influenced differently by the medium.

A preferred variant of the method according to the invention proposes that the first mode is a mode whose vibrations are essentially independent of the medium, and that the second mode is a mode whose vibrations are influenced by the medium become. Specifically, this means that a mode is selected as the first mode whose natural frequency or resonance frequency shifts as a result of a change in mass, but whose resonance frequency remains essentially unchanged when the oscillatable unit comes into contact with the medium. For the first mode, therefore, all modes come into question in which the cross-sectional areas 'vibration rods medium' of the vibration rods are small in the direction of vibration. If this requirement is met, then the interaction of the vibratable unit with the medium and thus the mass coupling of the oscillatable unit to the medium is relatively low. A mode is selected as the second mode, the natural frequency of which changes radically as soon as the oscillatable unit comes into contact with the medium.

An advantageous development of the method according to the invention provides that a change in the first mode, the vibrations of which are essentially independent of the medium, is used to identify whether a mass change has occurred on the vibratable unit. In particular, it is provided that, based on a change in frequency of the vibrations of the first mode, a build-up or a loss of mass on the vibratable unit is recognized.

While the first variant of the method according to the invention described above provides that two modes are selected which show completely different reactions as a result of the change in mass or as a result of contact with the medium, a second variant takes a different route. According to the alternative second variant of the method according to the invention, two modes are selected as the first mode and as the second mode of the vibrations of the oscillatable unit, both modes each having a first proportion which is dependent on the coupling to the mass of the medium, and both modes have a second component which is independent of the coupling to the mass of the medium and which is only dependent on the respective mass of the oscillatable unit.

An advantageous development of the method according to the invention provides that the functional dependence of the first and second modes of the vibrations of the vibratable unit on the medium or on the mass of the vibratable unit allows conclusions to be drawn about a change in mass of the vibratable unit. The only requirement with regard to the selection of the two modes is that they differ sufficiently from one another. The determination of the influence of the batch formation on the measured values is preferably carried out using a system of equations, which is composed of the two formulas mentioned below:

The symbols used in this system of equations indicate the following quantities:

ΔE _C : the relative frequency shift of a first mode; ΔE _D : the relative frequency shift of a second mode;

where the term

each symbolizes the relative frequency shift of the natural frequency of the corresponding mode, where relative means that the measured frequency shift is expressed in percent with respect to the corresponding natural frequency in air without formation. m _k : a measure of any type of mass coupling and damping through the medium. As already described in the previous section, the density p of the medium and the viscosity η of the medium also play a role here, in addition to the immersion depth h of the oscillatable unit. This can be expressed arithmetically by the following functional relationship: m _k = f (h; ρ, η); m _a : the starting mass; f _c (m _k ), f _ß (m _k ) the frequency shift curves of two sufficiently different modes (e.g. mode C and mode D) of the oscillatable unit as a function of the mass coupling m _{k of} the oscillatable unit and the damping of the oscillatable unit the medium (- »plunge curves); f _c ( ^m _a ) 'Ϊ _D ( ^tn Y the frequency shift curves of two sufficiently different modes (e.g. Mode C and Mode D) of the oscillatable unit as a function of the formation m _a on the oscillatable unit (→ - preparation curves). An advantageous embodiment of the method according to the invention provides that an error message is output when the frequency changes of a first and / or a second mode of the vibrations of the vibratable unit caused by the change in mass of the vibratable unit exceed a predetermined desired value.

It is particularly advantageous if a change in a first and / or a second mode of the vibrations of the vibratable unit caused by a change in mass of the vibratable unit is used to carry out an inline correction of the measurement data of the vibratable unit.

With regard to the device according to the invention, the object is achieved in that the control / evaluation unit uses at least a first mode and a second mode of the oscillations of the oscillatable unit for evaluation and that the control / evaluation unit recognizes a change in mass on the oscillatable unit based on the evaluated modes ,

An advantageous embodiment of the device according to the invention provides that the evaluation control unit is integrated in the device for determining and / or monitoring the fill level or for determining the density of the medium. In this case, the device according to the invention is a so-called compact sensor. The error message can e.g. B. be output optically, acoustically and / or digitally via at least two data lines.

An embodiment of the device according to the invention, which is an alternative to the compact sensor, provides at least two data lines via which the measurement data are routed to the evaluation control unit or via which the evaluation / control unit communicates with a remote control point. In this context, it is particularly advantageous if the respective measurement and / or correction data are transmitted digitally to the remote control point. Digital data communication has the known advantage of increased interference immunity compared to analog data transmission. For communication can of course use the known transmission protocols and transmission standards.

According to a preferred development of the device according to the invention, an output unit is proposed which optically and / or acoustically outputs an error message to the operating personnel if, preferably within the scope of predefined tolerance values, a predefined setpoint value for the frequency change is exceeded or undershot, which is due to a change in mass of the oscillatable ones Unity.

In addition, it is advantageously provided that the control / evaluation unit is assigned a memory unit in which soli values for tolerable frequency changes which are due to a change in mass are stored.

The invention is illustrated by the following drawings. It shows:

1: a schematic representation of the device according to the invention,

2: Possible, selected vibration modes of a preferred oscillatable unit with two paddle-shaped oscillating rods: a) Mode A, in which an occurring frequency change is influenced by the mass coupling to the medium, b) Mode B, in which an occurring frequency change essentially is based on formation, c) Mode C, in which the frequency change is influenced both by the formation and by the coupling to the mass of the medium, d) Mode D, in which the frequency change by both the formation and the coupling to the Mass of the medium is affected

3: sketched plunge curves of the modes A and B shown in FIGS. 2a and 2b with and without attachment mass and with a negative mass change, 4: sketched plunge curves of the modes shown in FIGS. 2c and 2d with and without attachment mass,

Fig. 5: schematic representation of the approach curves of different modes in air and

Fig. 6: a graphical representation of the frequency change tuple.

1 shows a schematic representation of the device 1 according to the invention for determining and / or monitoring the fill level of a medium in a container - by the way, the container and medium are not shown separately in FIG. 1. The device 1 shown in FIG. 1 is - as already explained at the previous point - both for filling level detection and for determining the density of what is in the container

Suitable medium. While in the case of level detection the vibratable unit 2 is immersed in the medium or not in the medium only when the detected limit level is reached, it has to be immersed in the medium continuously up to a predetermined immersion depth h for the purpose of monitoring or for determining the density p , The container can be, for example, a tank, but also a pipe through which the medium flows.

The device 1 has an essentially cylindrical housing. A thread 7 is provided on the outer surface of the housing. The thread 7 is used to fasten the device 1 at a predetermined fill level and is arranged in a corresponding opening in the container. It goes without saying that other types of attachment, e.g. by means of a flange that can replace screwing.

The housing of the vibration detector 1 is closed off by the membrane 5 at its end region projecting into the container 3, the membrane 5 being clamped into the housing in its edge region. The oscillatable unit 2 projecting into the container is attached to the membrane 5. In the case shown, the oscillatable unit 2 has the configuration of a Tuning fork, thus comprises two spaced-apart vibrating rods 3, 4 fastened on the membrane 5 and projecting into the container.

The membrane 5 is vibrated by a drive / receiving element 6, the drive element exciting the membrane 5 with a predetermined excitation frequency to vibrate. The drive element is e.g. B. a stack drive or a bimorph drive. Both types of piezoelectric drives are sufficiently known from the prior art, so that a corresponding description can be dispensed with here. Due to the vibrations of the

Membrane 5 also vibrates the oscillatable unit 2, the oscillation frequencies being different if the oscillatable unit 2 is in contact with the medium and is coupled to the mass of the medium, or if the oscillatable unit 2 is free and without contact with the Medium can swing.

As with the drive unit, the receiving unit can be a single piezo element, for example. The drive / receiver unit 6 excites the membrane 5 to vibrate as a function of a transmission signal applied to the piezo element; it also serves to receive and convert the vibrations of the membrane 5 into electrical reception signals.

Due to this vibration behavior of the piezoelectric element, the voltage difference causes the membrane 5 clamped in the housing to bend. The vibration rods 3, 4 of the oscillatable unit 2 arranged on the membrane 5 execute opposite vibrations about their longitudinal axis due to the vibrations of the membrane 5. Modes with opposite vibrations have the advantage that the alternating forces exerted by each vibrating rod 3, 4 on the membrane 5 cancel each other out. This minimizes the mechanical stress on the clamping, so that approximately no vibration energy is transmitted to the housing or to the fastening of the vibration detector. This can effectively prevent the fastening means of the vibration detector 1 from being excited to resonate vibrations again interfere with the vibrations of the oscillatable unit and could falsify the measurement data.

The electrical reception signals are forwarded to the control / evaluation unit 10 via data lines 8, 9. The control / evaluation unit 10 is assigned a memory unit 11, in which setpoints are stored which allow the control / evaluation unit to recognize a build-up on the oscillatable unit 2 and, if necessary, to have a corrective influence on the measured values. In the case shown, an error message is transmitted to the operating personnel via the output unit 14. Furthermore, the control or control center 12 arranged at a distance from the vibration detector 1 can be seen in FIG. 1. The control / evaluation unit 10 and the control point 12 communicate with one another via the data line 13. The communication is preferably carried out on a digital basis because of the increased interference immunity of the transmission.

FIGS. 2a, 2b, 2c and 2d show four selected and possible oscillation modes of an oscillatable unit 2 with two paddle-shaped oscillation bars 3, 4. In the mode B shown in FIG. 2b, the plunge curve ΔF is in the essentially independent of the mass coupling m _k to the medium, since the cross-sectional areas interacting with the medium are relatively small due to the oscillating movements occurring parallel to the paddle surface. The oscillation frequency is therefore essentially independent of the immersion depth h of the oscillatable unit 2 in the medium, but it shows a clear dependence on the attachment mass m _a present on the oscillating rods 3, 4. As already mentioned several times, analogous considerations also apply to a loss of mass that occurs on the oscillatable unit. Within certain tolerances, a change in frequency ΔF of mode B can therefore be used to draw a clear conclusion on the starting mass m _a present on the vibrating rods 3, 4.

This functional relationship can be seen graphically in FIG. 3. Fig. 3 shows the plunge curves ΔF (h) of the modes A and B shown in Fig. 2b with and without attachment mass m _a . The corresponding plunge curves ΔF (h) in the case of a negative change in mass are also shown in FIG. 3 the oscillatable unit 2, ie a loss of mass (m _k ) at the oscillatable unit 2; a loss of mass occurs e.g. B. due to corrosion or mechanical wear of the vibrating rods 3, 4. The immersion curves ΔF (h), that is to say the frequency change ΔF of the mode B as a function of the immersion depth h, have approximately the slope zero regardless of the mass of the oscillatable unit 2. So they run essentially parallel to the x-axis. Logically, the frequency change ΔF increases with increasing or falling mass change m _a . The immersion curves ΔF (h) of the mode A likewise shown in FIG. 3 show a completely different behavior: a change in frequency is clearly dominated here by the immersion depth h of the oscillatable unit 2 in the medium. Again, a positive or negative change in mass m _a , m _{k of} the oscillatable unit 2 is expressed in a parallel displacement of the plunge curves ΔF (h).

Both modes, Mode A and Mode B, are therefore ideally suited to be used in connection with a first embodiment of the method according to the invention. According to the first variant of the method according to the invention, the degree of formation (or the loss of mass) is determined using two modes, the first mode being a mode whose vibrations are essentially independent of the medium, and wherein it the second mode is a mode whose vibrations are essentially only influenced by the medium.

The frequency change .DELTA.F determined on the basis of the approach mass m _a (or the loss of mass) depends on the mode F - as proposed by an advantageous embodiment of the method according to the invention - for inline correction of the measurement data of the vibration detector 1. Furthermore, the information about the degree of build-up on the vibratable unit 2 or the mass loss of the vibratable unit

Unit 2 can also be used for 'Predictive Maintenance' purposes: The operating personnel is shown or informed when the oscillatable unit 2 must be cleaned or replaced by a unit 2 that does not have to be used.

FIGS. 2c and 2d show two further possible modes of an oscillatable unit 2 with two paddle-shaped oscillating bars 3, 4, which are preferably used in the second variant of the method according to the invention. The prerequisite here is that both modes C and D have a dependency on the mass coupling m _{k of} the oscillatable unit to the medium as well as a dependence on the starting mass that has formed on the oscillatable unit. Furthermore, the two selected modes must clearly differ from one another with regard to their plunge curves ΔF (h). That this is the case can be clearly seen from the diagrams shown in FIG. 4.

5 shows the approach curves .DELTA.F (of modes A, B and C). While mode B has only a slight dependence on the approach mass m _a , modes C and D show a strong dependence on a change in mass at the oscillatable unit 2 ,

The immersion curves ΔF (h) of the two modes C and D can be described mathematically formally in a first approximation (the mixed term is neglected) by the following system of equations:

c -f Xf a) (

_D -f _D H) + f) (2)

This system of equations must be solved for m _a = f (ΔF ₀ , ΔE. From equation (1) it follows:

f _c ² (m _a ) = AF _c - f ^ m _k ) (3)

From equation (2) it follows:

f _D ^l {m _k ) = AF _D - f _D ² {m _a ) ⁽ 4)

-1 m _k = f ^ {AF _D - f ^ (m _a ) (5)

Incidentally, a numerical solution method is preferably used.

From (3) and (5) we get:

By the way, there is no need to give an explicit formula for m _a , since ultimately only the relative frequency change of mode C that is caused by the formation of the approach is of interest. The limit value for / _c ² (m _a ) must be set such that a reliable detection of the predetermined fill level or the density of the medium is always guaranteed within the tolerable limits.

The preferably empirically determined immersion curves and attachment curves shown in FIGS. 4 and 5 can be approximated in a known manner by approximation functions and thus described mathematically.

Using the system of equations and the curves obtained by approximation, the value for / _c ² (m _a ), that is to say the relative frequency change of the mode C as a function of the starting mass m _a, can be determined for each measured frequency difference tuple ΔE _C , ΔE _D.

6, the measured values of the frequency difference tuples are plotted against ΔE _C , AF _D. The measuring points differ with regard to the immersion depth h and / or with regard to the attachment mass m _a formed on the oscillatable unit. The measuring points with the same attachment mass m _a are connected to one another in FIG. 7.

The measured values in the upper area of FIG. 6 represent the state of 'little batch mass', while the measured values in the lower area represent the

Represent the condition 'Much neck mass'. In order to evaluate the measurement data, it is therefore sufficient if the control / evaluation unit 10 measures the frequency changes of two sufficiently different oscillation modes, in the illustrated case mode C and mode D, and compares them with values that are stored in a table. The position of the measured values can then be used to clearly distinguish whether the build-up or loss of mass is still in the uncritical range or whether an alarm must be triggered. LIST OF REFERENCE NUMBERS

Vibration detector or density sensor

Viable unit

vibrating rod

vibrating rod

membrane

Exciter / receiver unit

thread

data line

Data line

Control / evaluation unit

storage unit

control Authority

data line

output unit

Claims

claims Method for determining and / or monitoring the level of a medium in a container or for determining the density of a medium in the container, wherein an oscillatable unit is attached to the level of the predetermined level or an oscillatable unit is attached so that it immersed in the medium up to a defined immersion depth, the oscillatable unit being by means of a Excitation vibration is excited to vibrate and the reaching of the predetermined fill level is recognized as soon as the vibratable unit vibrates with an oscillation frequency that has a predetermined frequency change compared to the excitation frequency, or the density of the medium is determined on the basis of the oscillation frequency of the oscillatable unit characterized in that at least a first mode and a second mode of the vibrations of the oscillatable unit (2) are evaluated and that a mass change in the oscillatable unit (2) is recognized on the basis of the evaluated modes. 2. The method according to claim 2, characterized in that modes are evaluated as the first and second modes, the vibrations of which are influenced differently by the medium. 3. The method according to claim 1 or 2, characterized in that the first mode is a mode whose Vibrations are essentially independent of the medium and that the second mode is a mode whose vibrations are essentially influenced by the medium. 4. The method according to claim 1 or 3, characterized in that on the basis of a change in the first mode, the vibrations of which are essentially independent of the medium, it is recognized whether a change in mass has occurred on the oscillatable unit (2). 5. The method according to claim 1, 3 or 4, characterized in that on the basis of a frequency change of the vibrations of the first mode it is recognized whether a mass change has occurred on the oscillatable unit (2). 6. The method according to claim 1 or 2, characterized in that two modes are selected as the first mode and as the second mode of the vibrations of the oscillatable unit (2), both modes each having a first proportion which is dependent on the medium, and wherein both modes have a second portion which is essentially independent of the medium and essentially only dependent on the respective mass of the oscillatable unit (2). 7. The method according to claim 6, characterized in that based on the functional dependence of the first and the second mode of the vibrations of the vibratable unit (2) on the medium or on the mass of the vibratable unit (2) conclusions on the mass of the approach, which has formed on the oscillatable (2) unit. 8. The method according to one or more of claims 1 to 5 or according to Claim 6 or 7, characterized in that an error message is output when the by the Changes in mass of the vibratable unit caused changes in the first and / or the second mode of the vibrations of the vibratable unit (2) exceed a predetermined target value. 9. The method according to one or more of claims 1 to 6 or according to claim 6 or 7, characterized in that a change in the mass of the vibratable unit (2) caused change in the first and / or the second mode of the vibrations of the vibratable unit ( 2) is used to carry out an inline correction of the oscillation frequency of the oscillatable unit (2). 10. Device for determining and / or monitoring the fill level of a medium in a container or for determining the density of a medium in the container, wherein an oscillatable unit is provided, which is attached to the level of the predetermined level or an oscillatable unit is attached in such a way that it is immersed in the medium to a defined immersion depth, a drive / reception unit being provided which excites the oscillatable unit with a predetermined excitation frequency to vibrate and which receives the oscillations of the oscillatable unit, and wherein a control unit / Evaluation unit is provided which detects when the predetermined fill level is reached as soon as a predetermined frequency change occurs or which determines the density of the medium on the basis of the oscillation frequency of the oscillatable unit, characterized in that the control / evaluation unit (10) has at least one first mode and a second fashion of Vibration of the oscillatable unit (2) is used for evaluation and that the control / evaluation unit (10) recognizes a change in mass on the oscillatable unit (2) based on the evaluated modes. 11. The device according to claim 10, characterized in that the evaluation / control unit (10) is integrated in the device for determining and / or monitoring the fill level or for determining the density of the medium. 12. The apparatus of claim 10 or 11, characterized in that at least two data lines (8, 9) are provided, via the measurement data to the evaluation / control unit (10) or via which the evaluation / control unit (10) with one remote control point (12) communicates. 13. The device according to one or more of claims 10 to 12, characterized in that an output unit (14) is provided which optically and / or acoustically outputs an error message to the operating personnel when, preferably within the scope of predetermined tolerance values, a predetermined target value Frequency change is exceeded or undershot, which is due to a change in mass at the oscillatable unit (2). 14. The apparatus of claim 10 or 13, characterized in that the control / evaluation unit (10) is associated with a memory unit (11) in the setpoints for tolerable frequency changes that go back to a mass change in the oscillatable unit (2), stored are. 15. The apparatus according to claim 12, characterized in that the measurement data and the data on the batch assignment of the oscillatable unit (2) are digitally transmitted to the remote control point (13).